1. Field of the Invention
The present invention relates to measurement of long distance or position such as measuring the traveling range of a frame of a truck crane, the position of a cylinder rod, the position of a carriage used in a vertical parking lot or an automated warehouse, the position of a container carrier, or the position of a top-running crane, and such as in a detector-equipped pneumatic cylinder or a tank liquid level meter.
2. Description of the Prior Art
There have been known some conventional methods of measuring a long distance; for example, a method using a radar or a laser beam, a technique for measuring the number of windings of a wire over a reel in which a winding detector is installed and also the angle of the winding to determine a position, and a technique for measuring the angle and the number of revolutions of a wheel which is equipped with a revolution detector and urged against a rail or the like to calculate the distance.
Such conventional distance measuring methods have disadvantages. More particularly, the accuracy of a radar method and a laser beam method may largely be affected by local climate such as sunlight levels or raining. In the method using a wire and a reel, the winding length per revolution of the wire may be varied due to change in the winding radius or rough winding. The method using a rail and a wheel may cause the wheel to slip and change its number of revolutions resulting in measurement error and in addition, its measuring of the revolution involves repeat (incremental counting) of the signal at each revolution hence requiring resetting of the origin when the measuring apparatus has been switched off and started again. Therefore, those will be less reliable in the continuous operation in any factory automated application or outdoor use.